World of Warcraft.
I still get *cold shivers* (haha) even though I stopped it in November -- been playing since first week of retail release. Sometimes I find myself twiddling thumbs, especially if on iTunes it runneth over from a previous album on to the WoW Lich King soundtrack. Might get Cataclysm to give it a whirl, but am not sure yet. Don't wanna get dragged in again lol.